This invention relates to nickel base superalloy articles possessing improved resistance to hydrogen embrittlement, and also improved fatigue resistance in air. The invention comprises a combination of aspects including composition, and heat treatment which produce an unexpected improvement in properties in certain environments. The invention primarily comprises a limited composition range in combination with a specific heat treatment and a HIP treatment. The result of this product has the unique combination of properties previously described.
The present invention deals with improvements to the hydrogen embrittlement resistance to fatigue failure of high strength nickel base superalloy single crystal articles. The invention aspects which provide the improvements to hydrogen embrittlement resistance also provide significant benefits to the fatigue behavior of the materials when used in an air atmosphere where gas turbines operate.
High strength nickel base superalloys are defined in the context of this invention as nickel base alloys containing more than about fifty volume percent of the strengthening xcex3xe2x80x2 phase and having a yield strength in excess of about 100 ksi at 1000xc2x0 F. Such alloys find their widest, and heretofore almost exclusive, application in the field of gas turbine engines. These alloys are also used in liquid hydrogen fueled rocket propulsion systems where hydrogen embrittlement is a limiting factor in the life cycle. In the development of the NASA Space Shuttle main engines, hydrogen embrittlement has been recognized to be a significant problem. The Space Shuttle main engines are rocket engines which mix and react liquid hydrogen and liquid oxygen to form the propellant. These reactants are pumped into the main combustion chamber by turbo pumps which are powered by the combustion products of the reaction of hydrogen and oxygen. The hot side of the turbo pumps, which is exposed to the combustion products of the hydrogen/oxygen reaction, includes a multiplicity of small turbine blades which are currently investment cast from either directionally solidified Mar-M 246 alloy or single crystal PWA-SP1493. (Nominal compositions shown in Table I). Both alloys meet the previous definition of a high strength nickel base superalloy in that they contain more than fifty volume percent of the xcex3xe2x80x2 phase and have a yield strength of more than 100 ksi at 1000xc2x0 F. Hydrogen embrittlement of these turbine blades is a problem of great concern and is one of the factors which requires the space shuttle main engine pumps to be rebuilt with substantially greater frequency than originally anticipated.
The initiation of hydrogen embrittlement cracking in nickel base single crystal superalloys is commonly observed at low temperatures (near 26 C.) and has been found to occur within the xcex3/xcex3xe2x80x2 eutectic islands where cracks form between the eutectic lamella. This is in contrast to air breathing gas turbine experience where cracks usually form at elevated temperature from a number of microstructural discontinuities such as pores, hard particles and interfaces between precipitated phases and the matrix.
According to the present invention, a class of nickel base superalloy compositions is described which can be processed to provide a high strength nickel base single crystal superalloy material which is in particular, highly resistant to hydrogen embrittlement. The principles taught in this invention also provide marked increases in the fatigue resistance of these alloys when used in more common applications, such as gas turbine engines.
The mechanism of the present invention is twofold:
(1) In the presence of high pressure hydrogen, i.e. in cryogenic rocket propulsion systems, the normally benign xcex3/xcex3xe2x80x2 eutectic islands inherent in Ni base turbine blade alloys become active fatigue crack initiators. Eliminating these eutectic islands thus significantly retards cracking in the presence of hydrogen and
(2) Microstructural features acting to reduce fatigue life in atmospheres other than high pressure hydrogen, namely those atmospheres encountered in gas turbine propulsion systems include hard carbide, boride and nitride particles, and, most significantly, microscopic interdendritic porosity. The elimination of these potential fatigue crack initiation sites thus significantly retards cracking in the gas turbine environment.
Maintaining levels of carbon, boron and nitrogen in the master melt as low as possible, and minimizing any introduction of carbon, boron or nitrogen during casting minimizes the amount of the hard phases which can form during the casting process.
The invention composition range, presented in Table II, in the as cast condition, contains a greatly reduced amount of eutectic xcex3/xcex3xe2x80x2 phase relative to the typical prior art alloys. The invention composition is also particularly suited for heat treatment to fully dissolve, and thereby eliminate, the eutectic xcex3/xcex3xe2x80x2 islands without causing incipient melting to occur in the alloy.
The invention also provides an optimum (Ni+Co)/Cr ratio, which provides maximum fatigue strength in both air and hydrogen. The ratio, (nickel+cobalt)/chromium in atomic percent in the xcex3 matrix phase, should be in the range of 1.5 to 3.0. We have found that controlling this ratio in this range is important in obtaining the maximum alloy tensile strength and fatigue strength.
Material which satisfies the compositional rules is solidified from the melt, then hot isostatic pressing is preferably used to close any porosity which might be present, and solution heat treatment at a temperature above the xcex3xe2x80x2 solvus temperature is conducted to dissolve the xcex3/xcex3xe2x80x2 eutectic islands without causing incipient melting. The article is then given conventional lower temperature heat treatments to produce a xcex3xe2x80x2 morphology which tailors the mechanical properties of the material to meet the requirements of the particular application. The resultant product is a high strength nickel base superalloy article which has significantly improved resistance to fatigue in air as well as resistance to hydrogen embrittlement.
The alloy composition and processing techniques disclosed and claimed herein are directed toward single crystal materials. Elements such as carbon and boron, which are known to form hard particles in single crystal materials and act as crack initiation sites, are substantially excluded from this invention. However, their presence is frequently required for grain boundary strengthening, so the teachings of this invention are not directly applicable to polycrystalline and columnar grain materials.
The foregoing and other features and advantages of the present invention will become more apparent from the following description and accompanying figures.